Secrecy system for signaling



Jan. M, i930. c. R. ENGLUND ET AL L743y70 SECRECY SYSTEM `FOR SIGNALINGFiled Nov. l, 1927 2 Sheets-Sheet 1 l n A 70m/Ey Jam 14? E930 C, R,ENGLUND Er AL 1,743,7@

SECRECY SYSTEM FOR SIGNALING Filed Nov. 1. 1927 2 Sheets-Sheet 2 ,rraSTABS ArsN'r f CABLQB. ENGLUND, 0F FBEEHOIJ, AND FREDERICK B.LLEWELLY'N, 0F MON'TCLAIB, NEW JERSEY, ASSIGNORS TO BELL TELEPHGNELABORATORIES, INCORPORATED, 0F

NEW YORK, N. Y., A CORPORATION 0F NEW YORK SEGREGY SYSTEM FOR SIGNALNGContinuation of application SerialNm 217,180, :Lied September 2, 1927.This application led November Y l 1, 1927. Serial No. 230,222. Y

This invention relates to Wave transmission, especially signaling byelectric waves. vAn object is to prevent unauthorized obtaining ofinformation carried by transmit- 5 ted waves.

A further object is to obtain such secrecy without unduly broadening therequisite width of the frequency band comprising the frequencies whichthe components of a wave transmitting information have in thetransmission medium.

In one specific aspect the invention is a radio `telephone system inwhich a speech wave separately modulates two carrier waves of suchfrequencies that the resulting upper side bands for example, arecontiguous and when selected and added form a band of fre uencies doublethe width of the speech ban the carrier waves being suppressed in themodulators.

This band of double the speech band width is caused to modulate avariable frequency carrier wave, which is suppressed in the modulator.The resulting upper side band, for example, is a wobbling band, itswidth in the frequency scale bein twice the width of the speechfrequency and at each instant, and its position in the frequency scalevarying in synchronism with the variation of the frequency of thecarrier wave, through a frequency "range of variation equal to that ofthe carrier wave.

This wobbling band is impressed on a filter which passes only afrequency band of the speech band width having lts center somewherebetween the frequency limits of. the w-cbbling band, as for example',midway between the frequency limits of the wolfbling band. The passedband is radiated from a transmitting antenna and received by .a distantreceivlng antenna. The pass 'band of the filter is`of constant width,and at `each instant the radiated band contains components representingall of the components of the speech waye, with their identity preserved.However, the components representing speech components are permuted inacyclic order in this radiated band. The band consists of two contiguoussub-bands each of varying width, the upper one con- Aband extendingnpwar taining components representing the components of a variableportion of the speech from the lower limit of the speech band, and thelower of the two sub-bands containing components represent- -ing theremaining components of the speech band. The information carried by theradiated band cannot be obtained from the band without knowledge ofvarious factors, including the frequency of the variable frequencycarrier wave.

The band incident on the receiving antenna is demodulated separately byytwo waves of variable frequencies, the frequency of one of these wavesbeing the algebraic sum of the variable carrier frequency and the lowerof the two fixed carrier frequencies employed at the transmit-ter, andthe frequency of the other variable carrier wave employed at thereceiver being the algebraic sum of the variable'carrier frequency andthe higher of the two fixed carrier frequencies employed at the sendingstation. This demodulation yields the two above mentioned variableportions of the speech band, which together comprise at each instant allof the components of the speech wave. The demodulations also yielddistortion coniponents, but certain of these are filtered out, and theremainder do not render the re ceived speech unintelligible.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

Fig. l of the drawing is a circuit diagram of the. sending stationreferred to above; Fig. 2 is a similar diagram of the receiving stationreferred to above; and Fig. 3 is a set of graphs for facilitatingexplanation of the invention.

In the following description, frequencies are giv'en in terms of radiansper second, except when otherwise indicated, and in symbolicalexpressions for current or voltage waves, coefficients indicatingamplitudes are omitteid, only quantities that indicate frequency beinggiven. Specific values of quantities such as frequencies, speeds, etc.,are mentioned merely by way of example, and the invention is not limitedthereto.

En Fig. 1 a signal wave, for example a speech wave from telephonetransmitter 1, is introduced Iinto the s ch or signal frequency inputcircuitV o' each of twomodulators lill- 2 and M+S. Each of the modullators ma`l be, for example, of the usual type of ba anced vacuum tubemodulator arranged for carrier wave suppression.

The carrier wave for modulator M-2 may be obtained from an'oscillator 05 of a frequency c which may be, fo'r example, 2n 20,000. f e

The carrier wave for modulator M may be obtained from an oscillator 0-'6of a frequency c-i-q, where lis'the difference between the highest Vandt e lowest frequencies of the speech wave that is to be transmitted.rlhus, if is 21r 3,000, then the frcquency of 'oscillator O-,-6 may be21|- (20,000+3,000). The lowest frequency of the speech band that is tobe transmitted may be designated so.

The upper side bands, for example, from modulatorsM-2 and M3 areselected by .filters BF- and BF-S respectively, and

amplified by amplifiers A`9 and A-lO respectively, and applied as amodulating .wave to. a modulator M-ll, which may be, for example, of theusual type of balanced vacuumtube modulator arranged for sup- ,pressionof the carrier wave. AThe filter BF-7 may be, for example, a band terpassing only frequencies between c-i-s., and c+s0-j-g. The filter BF--8may be, for eX- ample, a band filter passing only frequencies betweena+s0|g and c+s+2g. The modulating wave supplied to modulator M-ll fromamplifiers A-9 and A-lO is then two identical speech side bands oneimmediately/above the other, the two occupying the /frequency range fromG+S., to c+s+2g.

In modulator M-11 this wave modulates a variable frequency carrier wavesupplied from source O-12. 'The frequency of this carrier wave may varyin any suitable manner, as for example sinusoidally, ten times persecond, between a lower limiting frequency d and an upper limitingfrequency d -I- g Then, designating the value ofthis variable carrierfrequency la, the frequency h will vary about an average value d througha range g. The source O-12 may be, for example, a variable frequencyoscillatorof any suitable type. A suitable type is disclosed. in KendallPatent 1,571,010, January v26, 1926, or 1,592,940, July 20, 1926. Thefrequency d may be, for example 21r 20,000.

t The speech wave vthat is to be transmitted is assumed, as indicatedabove, to have a frequency spectrum extending from a fre- .quency so upto frequency gli-8. and is assumed to consist of two contiguous subbandsor portions, s1 being taken as representative of those frequenciesbetween so and Q-i-a) where :u may be a variable number and is neverless than 1, and .92 being taken as representative of the remainingfrequencies, which are those between (g+s) and gts.V The speech may thenbe symbolically Written cos slt-i-cos 8215, where, for

generality, s1 and s, may -exist either simultaneously or one at a time.

wave from aplifier A-9, and cos (p g-i-s?) t andcos ,(P+g+s2)t, both dueto the modulating wavefrom amplifier A-lO. As indicated in Fig. 3, thisside band represents two Variable frequency or wobbling side bandsexactly alike except relatively displaced in the frequency scale so thatthe lowest frequency of the side band (extending from p-i-g-i-s., up top7l-2g+s) due to the modulating Wave from amplifier A-lO is equal to thehighest frequency of the side band (extending from p-i-so up to p+g+s)due to the modulating wave from amplifier A-9.

The average value of frequency p will be designated pa. The band passfilter BF-20 passes only the frequencies between p., -I- g+so, andpa -I-2 That is, it passes only the frequencies lying l i 2 center frequency,pa-l-g+s. The frequency p may take on any values between radians persecond on either side of its pa-gand pa-ivBF-20 consistsof componentswhich may be symbolically Written cos (p-Il-s2)t and cos (p-t-g-I-s.

This output wave is amplified in any desired number of amplifiers A-21,radiated from the sending antenna 25' and received by the receivingantenna 30 at the distant receiving station, shown in Fig. 2. Althoughthe Wave contains componerfts representing all of the speech components,with the identity of each.

preserved, the information which it carries can not be obtained from itWithout knowledge of various factors, including the variable frequencyp, which is not transmitted.

From the antenna 30 the received Wave is impressed on the inputci'rcuits'of each of two demodulators DM-32 and DNI-33, which may be forexample, like modulators M--2 and M3.

The carrier or demodulating wave for demodulator DM-32 has the variablefrequenc p, and the demodulating Wave for demo ulator DM--33 has thevariable freyIn ther/system of the drawing, t ese demo ating Waves areobtained from variable frequency oscillators 0 45 and 0 46 like theoscillator O--l2 at the sendin r station. l y

he low fre uency output wave fromdemodulator D --32 may be symbolicallyWritten cos ezt-l-cos (g4-8,)t-i-cos (g-l-sl-sgt,

uency pfland the low frequency output wave from demodulator DM--33 maybe symbolically written,

cos (q s2) t cos sit cos (q s1 s2) t.

These waves, after amplication by amplifiers A-55 and A-56 respectively,are 1mpressed on low pass filter LPF-60, which has its output sideconnected to tele hone receiver 61, and which has a cut-off requencyg-i-su. The com onent cos (g4-81W is suppressed by the lter. Thecomponents slt and cos szt constitute all of the components of theoriginal speech wave, at their original speech frequencies, andreproduce the signal intelligibly in receiver 61, although the componentcos (g-s2) and the two components of frequency g-l-sl--sz are distortioncomponents. The two latter components are customarily present in speechdemodulators.

It will be apparent that the invention is not limited to radiotransmission but is applicable, for example, to Wire carrier systems andto systems in which the fre uencies employed for transmitting speec orother signals between two stations are within the frequency limits ofthe original frequencies of the signal components, the radiotransmission channel between the stations being representative of anysuitable type of transmission channel such as a voice frequency or wirecarrier frequency channel.

Although in the system shown in the drawing the waves of the frequencieso and c-l-g are generated by separate oscillators, it will be apparentthat, if desired, suitable means, as disclosed in Fig. 3 of H. W.Nichols Patent 1,545,270, July 7, 1925, may be employed for deriving thesecond wave from the first by modulating the first with a wave offrequency equal to the difference between the desired frequency of thesecond wave and the frequency of the first wave, and selecting the upperside frequency from It will be apparent that the frequencies of thecarrier waves at the transmitting station and the demodulating waves atthe receiving station may be kept in proper relation b suitableelectrical means, as for examp e, in the man-ner indicated in the abovementioned Kendall patents, Voin thc manner in which televisiontransmitting apparatus and receiving apparatus has been synchronized.Means which have been eniployed for synchronizing such televisionapparatus are disclosed for example in the copending applicatiton of H.M. Stoller and E. R. Morton, Serial No. 200,799, June 23, 1927.

If desired, oscillator O--45 can be a vacuum tube oscillator carefullyconstructed to have the same frequency variations as oscillator O-12,although in a different frequency range; or if preferred, the demodulating wave of frequency -p for demodulator DM-BQ may be obtained byhaving two oscillators (not shown) at the receiving station, identicalwith the oscillators O-5 and/O=-12 respectively at the sending station,and a modulator (not shown) for producing the frequency p from therWaves of such two oscillators. Similarly, the oscillator 0 46 can beconstructed to have the same frequency variations as oscillator O-12, ina different range; or the demodulating wave of frequency p-i-g can beobtained as aside band resulting from intermodulation of waves of twooscillators (not shown) identical -withoscillators 0 6 and O-lQrespectively.

VAs indicated above, it isnot necessary that the frequency ofoscillators 0 12, O45 and 0 46 vary throughout a frequency range equalin width to that from 8 to g|-s. With any smaller variation, if still ofconsiderable magnitude, the system will still afford a large element ofsecrecy, because all components of the signal undergo variation infrequency, in the Wave transmitted between the sending station and thereceiving station. If the frequencies of oscillators O-12, 0-45 and 0 46vary less than q radians, then the pass band of filter Blf-20 does nothave to be located exactly midway in the extreme side band variationrange but may be in any position in that range, such that the speechside bandsdo not materially slide off the characteristic selective rangeof the filter. The rate of variation of the frequencies of oscillatorsO`-12, 0 45 and 0 46 preferably should be sufficiently high to preventsyllable reception W-ith use of a constant beating frequency.

' Preferably the frequency of variation is k'between about cycles persecond and about 15.

cycles .per second. As indicated above,

cycles alsecond is a very satisfactory frequency of variation. It willbe apparent that the variation-need not be'sinusoidal, and that ifr,desired it may be intermittent ratherthan continuous, and moreover,that the mode of the variation of frequency may be changed, in anysuitable manner agreed upon between the operators at the twofstatioris,l to still further increase the secrecy of transmission.

Y For simplicity the drawing shows the ilaria# tion 'as sinusoidal; butfor greater secrecy it is preferable to havethe variation irregularwithinY each cycle of variation. Y Y

Although the ,specific value of frequency d mentioned above byway ofexample is equal tothe value mentioned for frequency c, such 'A art thatvarious modifications may be made in the system shown in the drawing,Without departing from the invention. Thus, for example, the filterBld-20 may be omitted if the antenna-25 is tuned suficiently sharply tosuppress the frequencies outside of the band which it is desired toradiate.

This application is a continuation of our application Serial No. 217,130, filed Septemberr2, 1927, entitled l Secrecy systems for signaling.

What is claimed is:

1. The method of operating on a Wave having components of differentfrequencies,. relatively displacingv which comprises groups of saidcomponents in the-frequency scale-and progressively varying the widthvof the frequency uband formed by the frequencies of the components thatform one -of the relatively displaced groups.

, 2. 'The method which comprises changing the order of sequence ofportions of a band of frequencies in the frequency scale and varying thewidths of said portions.

3. The method which comprises transposing portions of a band offrequencies and cyclically and continuously varying the width of each ofsaid portions while maintaining their combined width constant.

4. The method which comprises so variably permuting, in the frequencyscale, currents vhaving frequencies forming sub-bands that form afrequency band, as to vary the widths of the sub-bands and maintain allof the permuted currents Within a frequencyv .band width equal to thatofthe lirst nientioned band at each instant.

5. The method which comprises so varying the frequency of each of thecomponents of a band of signal com onents of dierent frequencies as toproduce from the first mentioned band a frequency band of the signalcomponents which has the sainev Width as the first vmentioned band, andwhich contains the signal components of the first mentioned bandvariably permuted in the frequency scale but at each instant all presentthat are present `in the yfirst mentioned band at a correspondinginstant, and Wliieh'iscomposed of variable 'widthL sub-bands Vof thesignal components throughout each of whicli'the signal components havetheir original order of sequence in thel frequency scale.

6. Themethod which comprises relatively displacing, in the frequencyscale, groups of Wave lcomponents that form portions of a band offrequenciesy and gradually varying thewidth of one of the relativelydisplaced portions, transmitting the relatively disf placed portions toa distantl point, and at that point varying the frequencies of thecomponents transmitted. and restoring the portions to their initialrelation.

y 7. The vmethod of transmitting signal components of differentfrequencies comprised in a frequency band, which comprises transposinggroupsof the components that form portions of the band in the frequencyscale sand repeatedly varying the width of certain of the frequencybands formed by the transposed groups, fory transf mission of the signalcomponents, and, for

reception of the signal components, restoring the transposed groups totheirinitial relative positions inthe'frequency scale.

8. The method of operating on a plurality of frequency bands ofcomponents, eachA band varying in width but all of the bands formingduring said` variation a band of constant width, which comprises varyingthe frequency of each ofthe components of the first mentioned bands andtransposing the irst mentioned bands inthe frequency scale.

` 9. `In combination, means for producing tw contiguous frequency bandsof signal f components with each band representing the same signal, andmeans for varying the frequencies of certain of ,the components of eachband.

10. In combinationmeans for producing two contiguous frequency bands ofsignal components with each band representing the same signal and withthe signal components of eachl band varying in frequency, and means forselecting two' such contiguous, variable width, frequency groups o f thecomponents, one group from each baud, as to thereby form a frequencyband of the components that is constant in 4width and that' has constantfrequency limits.

those at the lower end when the frequency of the carrier wavesincreases, and vice versa. 12. In combination, means for producing twocontiguous frequency bands of signal components witheach bandrepresenting the same signal and with the signal components of each bandvarying in frequency, and means for selecting two contiguous frequencygroups of the components, one group from each of the two bands.

13. In combination, means for producing two contiguous frequency bandsof signal components with each band representing the same signal andwith the signal components of each band varying in frequency, and meansfor selecting two such contiguous frequency groups of the components,one group from each band, as to thereby form a frequency band of thecomponents that is constant in width.

14:. 4In combination, means for producing two contiguous frequency bandsof signal components with each band representing the same signal andwith the signal components of each band varying in frequency, and meansfor selecting two contiguous frequency groups of the components, withthe components of each group forming a frequency band of varying width.

15. A secrecy signaling system comprising a transmitting station havingmeans for producing from the signal to be transmitted a wave in whichcomplementary portions of the signal frequency band each of which variesin width are reversed as to their order of sequence in the frequencyscale, and a receiving station having means for restoring the portionsto their original positions in the fre uency scale.

16. pparatus for receiving signal wave components forming a plurality offrequency bands each varying cyclically in width, said apparatuscomprising means for cyclically varying the frequency of each of thecomponents at the frequency of said cyclical variation of band width andtransposing the bands of variable width in the frequency scale to form aresulting wave, and means for selecting from the resulting wave afrequency grou of wave com onents and reproducing t e signal there romin a form intelligible to one of the senses.

17. In a method of receiving a plurality of frequency bands each varyingcyclically in width, cyclically varying the frequencies of thecomponents at the frequency of said cyclical variations of band widthand transposing the bands of variable width in the frequency scale.

18. A method of operating on mutually exclusiveu portions of a band ofsignal components of various frequencies, which portions are variable inwidth in the frequency scale in such a manner that the respectivefrequency ranges over which said portions extend, at least partlyoverlap, said method comprising varying the frequencies of thecomponents, synchronously with the first mentioned variations andthrough a frequency range of variation equal to the range of the firstmentioned variations, and transposing the bands of variable width in thefrequency scale.

19. In combination, means for producing two contiguous frequency bandsof signal components with each band representing the same signal andwith the signal components of each band varying in frequency, and meansfor selecting two contiguous frequency groups of the components, onegroup from each of the two bands, and each group of variable width inthe frequency scale.

20. In combination, means for producing two 'contiguous frequency bandsof signal components with each band representing the same signal,` meansfor selecting two contiguous frequency groups of the components, onegroup from each of the two bands, and means for varying the frequenciesof the components of each group.

In witness whereof, we hereunto subscribe our7names this 31st day ofOctober, A D. 192

CARL R. ENGLUND. FREDERICK B. LLEVELLYN.

